Infectious diseases consultant Dr Martin Wiselka said: "Damage will be done if raw sewage starts to infect water. If water supplies have been contaminated it will pose a serious, and possibly fatal, problem."

Residents are also at risk of catching cryptosporidiosis - a bowel infection common in third world countries with symptoms including diarrhoea, sickness and fever lasting for up to a month.

Dr Wiselka added: "In healthy people the disease would be very uncomfortable but it could turn life-threatening for vulnerable people.

"Babies, the elderly, those with a lowered immune system could all be at risk."

Water-borne disease expert David Harper said it is vital people are wary - adding if temperatures rise, water could become infested with mosquitos which can spread malaria and meningitis.

The public health consultant, 71, said: "E.coli would be a major concern as that can enter the water from human or animal faecal matter or dead animals. Weil's Disease, spread by rat urine, is also a potential danger.

"The disease is passed through the blood so those with cuts could become infected.

"And to be frank, this disease can be fatal, especially for younger and elderly people."

The shock findings came to light after microbiologist Nathaniel Storey, from the University of Reading, took samples from moorland and found the water contained 60,000 to 70,000 bacteria per 100 millilitres.

The World Health Organisation says agricultural water should have no more than 1,000 bacteria per 100 millilitres.

Water treatment expert Michael Pritchard warned that hundreds of Somerset families who rely on water from wells could face contamination from E.coli.

He explained: "Several hundreds of families will be on their own wells, so they definitely should not drink it."

Mr Pritchard also suggested there could be an increase in cases of Weil's disease.

He added: "As the water rises and the sewerage rises and the rats are forced out of it, that sort of contamination will spread."

Last night high tides and gale force winds were set to bring more flooding to parts of south-west England and the Midlands.

Environment Secretary Owen Paterson urged residents to "listen to all the advice being issued".